1. Field of the Invention
The present invention generally relates to an image reading apparatus and an image reading method, and more particularly relates to the technique of reducing the time required for shading compensation and improving the throughput of reading document sheets.
2. Description of the Background Art
In an image reading apparatus, shading compensation is performed for the purpose of preventing an image from being unevenly read due to variations in the light intensity of a light source lamp and time-dependent change thereof, the sensitiveness dispersion of image reading elements and so forth. The shading compensation is performed by sampling a white reference plate provided in the image reading apparatus with a width which is greater than or equal to the main scan width to acquire white reference values and prepare compensation data for each picture element. Then, the image of a document sheet is captured followed by compensating the densities of the image in accordance with the compensation data.
Japanese Patent Published Application No. 2002-33915 discloses a simplified shading compensation technique which avoids throughput degradation which results from the step of sampling a white reference plate each time the document sheet is read. In this simplified shading compensation technique, when a first document reading process is performed, the white reference plate is sampled to perform ordinary shading compensation, but the read data of the subsequent document reading process is compensated by measuring the light intensity of the light source lamp without sampling the white reference plate, on the basis of the reduction of the light intensity from the light intensity measured when the first document reading process is performed.
Generally speaking, while a small auxiliary white reference plate is provided separate from the white reference plate, the light intensity of the light source lamp is measured by reading the small auxiliary white reference plate by some of image reading elements. In the case of the image reading apparatus provided with an automatic document feeder which is capable of reading document sheets without need for moving a light source lamp, the throughput of reading document sheets can be further improved by providing an auxiliary white reference plate located in a position where it can be read without need for moving the light source lamp from the document reading position.
The simplified shading compensation technique described in Japanese Patent Published Application No. 2002-33915 is performed by the use of the white reference values which are acquired when the first document reading process is performed, and then uniformly adjusted in accordance with the reduction of the light intensity after the first document reading process.
However, the reduction of the light intensity sometimes varies along the main scanning direction due to the characteristics of the light source lamp, the differences of temperature in the vicinity of the light source lamp under the influence of the airflow, for example, generated by cooling fan, and so forth. Because of this, the technique of uniformly adjusting the sample values of the white reference plate measured when the first document reading process is performed based on the reduction of the light intensity cannot always be correctly adjusted in proportion to the actual reduction of the light intensity.